1. Field of Invention:
This invention relates to processes for the dealkylation of alkyl aryl ethers to aryl phenols and more particularly to such processes using inorganic acids containing selected accelerators.
2. Prior Art:
A variety of reagents have been used to dealkylate alkyl aryl ethers. Commonly used reagents include hydrobromic acid, boron tribromide, organic mercaptides, and trimethylsilyl halides. The cleavage of ethers has recently been reviewed [M. V. Bhatt and S. U. Kulkarni, Synthesis, 249 (1983)].
While these standard methods of dealkylation have proven acceptable for a variety of simple ethers, the rate of dealkylation of more complicated ethers is often sufficiently slow that rather harsh reaction conditions or long reaction times are necessary to effect complete dealkylation, resulting in extensive decomposition and hence lower yield of the desired dealkylated product.
Canadian Pat. No. 913077 issued on Oct. 24, 1972 to Endo Laboratories, Inc., discloses the preparation of a variety of N-substituted 14-hydroxydihydronormorphines by dealkylation of the corresponding 3-methyl ethers. This dealkylation is exemplified by reaction with pyridine hydrochloride at 190.degree.-195.degree. C. for 75 minutes. No yield is given for this dealkylation. Other dealkylation reagents include hydrogen chloride, hydrogen chloride in acetic acid, hydrogen chloride in the presence of zinc chloride, ferric chloride, or antimony trichloride, hydrogen bromide, hydrogen bromide in acetic acid, hydrogen iodide, etc.
The need exists for a method of accelerating the cleavage of alkyl aryl ethers which react only sluggishly under simple acidic conditions without increasing the temperature or time of the reaction sufficiently to result in extensive decomposition.